1. Field of the Invention
The present invention relates generally to an apparatus for optically measuring the roughness of a surface, and more particularly to an optical surface roughness measuring apparatus which operates according to the heterodyne interference wherein a phase of a beat beam produced by a measuring laser beam and a reference laser beam changes with a change in the length of the optical path of the measuring laser beam, which occurs due to the roughness of the subject surface irradiated by the measuring laser beam.
2. Discussion of the Prior Art
As a surface roughness measuring apparatus, there is known an optical apparatus, in which the subject surface is irradiated by a laser beam, and the surface roughness is determined based on the frequency of the laser beam which changes due to the Doppler effect, as the surface roughness condition varies. Thus, the apparatus is capable of measuring the surface roughness without contacting the subject surface. For instance, the apparatus uses a laser source which produces two linearly polarized laser beams which have mutually perpendicular planes of polarization. These laser beams are split by a polarizing beam splitter or other suitable means, into a reference laser beam (having a frequency fr) which is incident upon a stationary mirror, and a measuring laser beam (having a frequency fm) which is incident upon a surface of a subject, perpendicularly to the subject surface. The subject is fixed on a suitable movable support member such as a rotary table or an X-Y table, so that the subject is moved to change the spot of the subject surface which is irradiated by the measuring laser beam. While the frequency fr of the reference laser beam reflected by the mirror is held constant, the frequency fm of the measuring laser beam reflected by the subject surface is changed according to the Dopper effect, as the surface roughness condition on the irradiated spot is changed with the movement of the subject, namely, as the spot irradiated by the measuring laser beam is displaced in the vertical direction. Therefore, the roughness of the subject surface may be determined based on the amount of change in the frequency fm of the measuring laser beam with respect to the frequency fr of the reference laser beam.
Thus, the known surface roughness measuring apparatus described above requires a movable support member for moving the subject, so that the irradiated spot is moved to measure the surface roughness. Further, the measurement of the surface roughness is influenced by undesirable movements of the subject itself, which arise from vibrations of a drive motor to move the support member, pitching and rolling movements and precession of the support member.
Further, the known apparatus suffers from a measuring error which is caused by two different optical paths along which the reference and measuring laser beams are propagated. Namely, the reference and measuring laser beams experience different optical characteristics of the optical systems which define the above-indicated two optical paths, and are subject to different influences by external conditions such as vibrations, and density and temperature of the surrounding atmospheres through which the two beams are propagated.